Im going to buy a agp graphic card?

Before you install any kind of hardware you need to learn the basics. The graphics card doesnt care about system memory, but it has its own memory on the card. What you need to pay very careful attention to is your AGP slot and what the card requires. If the card requires an AGP 8x pro and you have an AGP 3x-8x card, then the card and your computer are not compatible. Trying to use an 8x pro card in your slot could damage both the card and your computer.